1. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to transmission power control of sounding reference signals.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys signals from one or more base stations (NodeBs) to User Equipments (UEs), and an UpLink (UL) that conveys signals from UEs to one or more NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile, and may be a device such as a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, etc.
A communication system also supports several signal types of transmissions including data signals conveying information content, control signals enabling proper processing of data signals, and Reference Signals (RS), also known as pilots, enabling coherent demodulation of data signals or control signals or providing Channel State Information (CSI) corresponding to an estimate of a channel medium experienced by their transmission.
UL data information is conveyed through a Physical UL Shared CHannel (PUSCH). UL Control Information (UCI) is conveyed through a Physical UL Control CHannel (PUCCH), unless a UE also transmits a PUSCH, in which case the UE may convey at least some UCI in a PUSCH. UCI includes ACKnowledgment (ACK) information associated with a Hybrid Automatic Repeat reQuest (HARM) ACK (HARQ-ACK) process and is transmitted in response to receiving, by a UE, data Transport Blocks (TBs). UCI also includes DL CSI that informs a NodeB of a channel medium experienced by a signal transmission to a UE. An UL RS can be used to demodulate data or control signals, in which case the UL RS is referred to as DeModulation RS (DMRS), or to sound an UL channel medium and provide NodeBs with UL CSI, in which case the UL RS is referred to as Sounding RS (SRS).
DL data information is conveyed through a Physical DL Shared CHannel (PDSCH). DL Control Information (DCI) is conveyed through respective Physical DL Control CHannels (PDCCHs). A PDCCH can convey a Scheduling Assignment (SA) for PUSCH transmission from a UE (UL SA) or for PDSCH reception by a UE (DL SA).
FIG. 1 is a diagram illustrating a conventional PUSCH structure over a Transmission Time Interval (TTI).
Referring to FIG. 1, a TTI includes one subframe 110, which includes two slots. Each slot 120 includes NsymbUL symbols used to transmit data information, UCI, or RS. Each symbol 130 includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in one slot may be at a same or at a different BandWidth (BW) than the transmission in the other slot. Some PUSCH symbols in each slot may be used to transmit a DMRS 140. The transmission BW includes frequency resource units referred to as Resource Blocks (RBs). Each RB includes NscRB or Resource Elements (REs). A UE is allocated MPUSCH RBs 150 for a total of MscPUSCH=MPUSCH·NscRB REs for a PUSCH transmission BW.
In FIG. 1, the last subframe symbol may be used to transmit SRS 160 from at least one UE. Then, NsymbPUSCH=2·(NsymbUL−1)−NSRS subframe symbols are available for data/UCI/DMRS transmissions, where NSRS=1 if the last subframe symbol is used to transmit SRS, and NSRS=0 otherwise. An SRS transmission from a UE may be sent periodically at predetermined subframes with transmission parameters configured to a UE by higher layer signaling, such as Radio Resource Control (RRC) signaling, or it may be sent aperiodically and triggered by an UL SA or a DL SA.
A PUSCH transmission power is determined so that the associated signals are received with a desired Signal to Interference and Noise Ratio (SINR) at serving NodeBs while controlling interference to neighboring cells thereby achieving reception reliability targets and ensuring proper network operation. Open-Loop (OL) Transmission Power Control (TPC) with cell-specific and UE-specific parameters is typically used together with Closed Loop (CL) corrections through TPC commands from one or more serving NodeBs.
If a PUSCH is scheduled by an UL SA, a respective TPC command is included in the UL SA. If a PUSCH is scheduled according to Semi-Persistent Scheduling (SPS), where a UE is configured by higher layer signaling of a set of parameters for periodic PUSCH transmissions, a TPC command is provided by a separate PDCCH that provides TPC commands to potentially multiple UEs. For each PDCCH, a type of the PDCCH is identified by a scrambling applied to a Cyclic Redundancy Check (CRC) included in a PDCCH codeword. For DL SAs or UL SAs, a CRC is scrambled with a Cell Radio Network Temporary Identifier (C-RNTI). For a PDCCH providing PUSCH TPC commands, a CRC is scrambled by a TPC-PUSCH-RNTI. A PDCCH may also provide PUCCH TPC commands and a respective CRC is then scrambled by a TPC-PUCCH-RNTI. The scrambling operation may be an exclusive OR (XOR) operation, defined by: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0.
FIG. 2 is a block diagram illustrating a conventional transmitter block diagram for data in a PUSCH.
Referring to FIG. 2, data bits (and possibly UCI data bits) 210 are provided to a Discrete Fourier Transform (DFT) unit 220A mapper 230, which performs sub-carrier mapping, maps the output of the DFT unit to REs of a PUSCH transmission BW as indicated by selection unit 240, which controls transmission BW. An Inverse Fast Fourier Transform (IFFT) is subsequently performed by IFFT unit 250, a CP is inserted by CP insertion unit 260 and the resulting signal is filtered by filter 270 for time widowing. Finally, a transmission power (PPUSCH,c) is applied by power amplifier (PA) 280 and the resulting signal 290 is transmitted.
A UE can derive a PUSCH transmission power PPUSCH,c(i), in deciBels per milliwatt (dBm), in a serving cell c during subframe i, such as in Equation (1), where for simplicity it is assumed that a UE does not transmit both PUSCH and PUCCH in a same subframe.
                                          P                          PUSCH              ,              c                                ⁡                      (            i            )                          =                  min          ⁢                                    {                                                                                                                                            P                                                      CMAX                            ,                            c                                                                          ⁡                                                  (                          i                          )                                                                    ,                                                                                                                                                          10                        ⁢                                                                                                  ⁢                                                                              log                            10                                                    ⁡                                                      (                                                                                          M                                                                  PUSCH                                  ,                                  c                                                                                            ⁡                                                              (                                i                                )                                                                                      )                                                                                              +                                                                        P                                                                                    O                              ⁢                                                                                                                          ⁢                              _                              ⁢                                                                                                                          ⁢                              PUSCH                                                        ,                            c                                                                          ⁡                                                  (                          j                          )                                                                    +                                                                                                    α                            c                                                    ⁡                                                      (                            j                            )                                                                          ·                                                  PL                          c                                                                    +                                                                        Δ                                                      TF                            ,                            c                                                                          ⁡                                                  (                          i                          )                                                                    +                                                                        f                          c                                                ⁡                                                  (                          i                          )                                                                                                                                }                        ⁡                          [              dBm              ]                                                          (        1        )            wherein Equation (1):                PCMAX,c(i) is a maximum UE transmit power configured to a UE by higher layer signaling and may depend on a UE power amplifier class.        MPUSCH,c(i) is a PUSCH transmission BW expressed in number of RBs.        PO—PUSCH,c(j) controls a mean received SINR at serving NodeBs and is the sum of a cell-specific component PO—NOMINAL—PUSCH,c(j) and a UE-specific component PO—UE—PUSCH,c(j) that are provided to a UE by higher layer signaling for j=0 and j=1.        For j=0 or j=1, αc∈{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} is a 3-bit parameter provided to a UE by higher layer signaling. For j=2, αc(j)=1. Fractional TPC applies for αc<1 as the Path Loss (PL) is not fully compensated. For αc=1, pure CL TPC is provided.        PLc is a DL PL estimate, in dBs, a UE calculates as PLc=referenceSignalPower−higher layer filtered RSRP, where referenceSignalPower is provided to a UE by higher layer signaling and corresponds to a transmission power of a DL RS a UE measures to determine a PL. RSRP corresponds to a RS Received Power (RSRP) a UE measures using a filter configuration provided by higher layer signaling. A serving cell c for a PL measurement is configured to a UE by higher layer signaling using a parameter pathlossReferenceLinking.        ΔTF,c(i) either equals 0 or is determined by a spectral efficiency of a PUSCH transmission. Further details are omitted for brevity, as they are not material to the present invention.        fc(i)=fc(i−1)+δPUSCH,c where δPUSCH,c represents a TPC command included in an UL SA scheduling a PUSCH transmission or in a separate PDCCH providing TPC commands to a group of UEs. The TPC command may be accumulative or absolute.        
For SRS transmission in a serving cell c during subframe i, an SRS transmission power PSRS,c follows a PUSCH transmission power, such as in Equation (2):PSRS,c(i)=min{PCMAX,c(i),PSRS—OFFSET,c(m)+10 log10(MSRS,c)+PO—PUSCH,c(j)+αc(j)·PLc+fc(i)}[dBm].  (2)
In Equation 2:                PSRS—OFFSET,c(m) is a 4-bit parameter configured to a UE by higher layer signaling where for periodic SRS transmission m=0 and for aperiodic SRS transmission m=1.        MSRS,c is a SRS transmission BW expressed in number of RBs.        
A UE transmits DMRS or SRS by transmitting a respective Zadoff-Chu (ZC) sequence. For a UL system BW including NRBmax,UL RBs, a sequence ru,v(α)(n) can be defined by a Cyclic Shift (CS) α of a base sequence ru,v(n) according to ru,v(α)(n)=ejαn ru,v(n), 0≦n<MscRS, where MscRS=mNscRB is the length of the sequence, 1≦m≦NRBmax,UL, and ru,v(n)=xq(n mod NZCRS), where a qth root ZC sequence is defined by
                    x        q            ⁡              (        m        )              =          exp      ⁡              (                                            -              jπ                        ⁢                                                  ⁢                          qm              ⁡                              (                                  m                  +                  1                                )                                                          N            ZC            RS                          )              ,0≦m≦NZCRS−1 with q given by q=└ q+½┘+v·(−1)└2 q┘ and q given by q=NZCRS·(u+1)/31. The length NZCRS of a ZC sequence is given by the largest prime number such that NZCRS<MscRS. Multiple RS sequences can be defined from a single base sequence through different values of α. A UE implicitly determines a ZC sequence to use for DMRS or for SRS transmission through an identity NIDTP of a respective TP.
FIG. 3 is a block diagram illustrating a conventional transmitter block diagram for a ZC sequence.
Referring to FIG. 3, a mapper 320, which performs sub-carrier mapping, maps a ZC sequence of length MscRS 310 to REs of an assigned transmission BW as the Res are indicated by RE selection unit 330, which controls of transmission BW. The mapping can map the ZC sequence to consecutive REs for a DMRS or to alternate REs for a SRS thereby creating a comb-type spectrum. Subsequently, an IFFT is performed by IFFT unit 340, a CS is applied to the output by CS unit 350, a CP is inserted by CP insertion unit 360, and the resulting signal is filtered by filter 370 for time windowing. Finally, a transmission power (PPUSCH,c for a DMRS or PSRS,c for a SRS) is applied by PA 380 and the resulting RS transmission signal 390 is transmitted.
Improving coverage and cell-edge throughput are key objectives in communication systems. Coordinated Multi-Point transmission/reception (CoMP) is an important technique utilized to achieve these objectives. CoMP operation relies on the fact that when a UE is in a cell-edge region, it may be able to reliably receive signals from a first set of NodeBs (DL CoMP) and reliably transmit signals to a second set NodeBs (UL CoMP). DL CoMP schemes can range from simple schemes involving interference avoidance, such as coordinated scheduling, to more complex schemes requiring accurate and detailed CSI such as joint transmission from multiple NodeBs. UL CoMP schemes can also range from simple schemes where PUSCH scheduling is performed by a single NodeB to more complex schemes where received signal characteristics and generated interference at multiple NodeBs are considered. Herein, NodeBs for DL CoMP are referred to as Transmission Points (TPs) while NodeBs for UL CoMP are referred to as Reception Points (RPs).
FIG. 4 is a diagram illustrating a conventional DL CoMP operation.
Referring to FIG. 4, TP0 410 and TP1 420 are connected through a fiber optic link 430, which enables information exchange with negligible latency. A UE 440 receives a first signal 450 from TP0 410 and a second signal 460 from TP1 420, where the first and second signals 450 and 460 convey same information. Depending on a DL CoMP scheme, a combination of these two signals at a UE may be transparent or non-transparent. FIG. 4 may also be used to describe UL CoMP operation where TPs are instead viewed as RPs, transmissions of signals from TPs are instead viewed as receptions of signals from RPs, and a signal reception from a UE is instead viewed as a signal transmission.
Support of UL CoMP introduces new TPC requirements for PUSCH, PUCCH, and SRS. SRS transmission power control is also relevant for DL CoMP in Time Division duplex (TDD) systems where, due to the DL/UL channel reciprocity, a SRS may be used to obtain more accurate CSI over conventional CSI feedback from a UE based on DL RS measurements.
Therefore, there is a need to define an SRS transmission power control method to support UL CoMP or DL CoMP. There is also a need to decouple a PUSCH TPC process and a SRS TPC process to support UL CoMP or DL CoMP. Finally, there is also a need to provide different TPC commands for a PUSCH transmission power and for a SRS transmission power to support UL CoMP or DL CoMP.